1. Field of the Invention
The invention relates generally to optical measuring techniques which employ spectrophotometers and the like. Specifically, it relates to the measurement of dielectric film thickness by observing the variation of interference effects produced by varying the wavelength of the incident light.
2. Description of the Prior Art
A technique commonly used for measuring the thickness of a thin dielectric film (e.g., silicon dioxide, silicon nitride, or photoresists used on silicon in the fabrication of integrated circuits) is to observe the manner in which the intensity of light reflected from the film and its substrate varies due to optical interference effects as the wavelength of the incident light is varied. For given film and substrate materials, the distance between adjacent maximum or minimum points on the reflectance curve is theoretically a function of film thickness. In practice, however, there are likely to be deviations from the theoretical relationship due to the variation of the indices of refraction of the film and substrate with optical wavelength. In addition, unless the spectrophotometer is fully corrected for spectral variation of its light source, aging of its electronic components, mechanical wear, and calibration of its optical paths, further deviations from theory occur. In the conventional measurement, the locations of the maximum and minimum points are read by a human from the curve plotted by the spectrophotometer. These data points are then used to calculate the thickness of the dielectric film. While this method only requires relative reflectivity (rather than absolute reflectivity) in order to make the measurement, it requires intervention of a human to find the maximum and minimum points (with the associated unreliability and inaccuracy of human observers) and also utilizes only a small amount of the measured data while demanding equal accuracy at all points. In the conventional spectrophotometric system used for this purpose, the optical apparatus has numerous electrically and mechanically adjustable parts which must be adjusted both periodically and aperiodically to compensate for system variations of the kind mentioned above. All of these adjusting, compensating and calculating functions customarily are accomplished by time-consuming human and mechanical operations, which greatly impede the measurement of film thicknesses.